## How to Change Areas of Truss Elements

You can always increase the FOM of a truss by increasing the areas
of the highest stressed elements and decreasing the areas
of the lowest stressed elements or both. Ideally, all elements
should have the design stress.
Remember, you are building a model. The small increase in weight
caused by the increase in area of a few elements is unimportant
since the model will be scaled up to full size.
If you find that after you change areas, the FOM stays
the same or decreases, you made a mistake.

After running Ansys on a truss,
Change the cross-sectional area of each element
so its stress will be close to **16 ksi**. Increase the area of those
elements with stress greater than 16 ksi and decrease the area of
those with stress less than 16 ksi by defining new R statements
in your program. Since several elements will have the same stress
because of symmetry, you only need a few R commands. Do not use
duplicate R values.

Since we do not know the elastic modulus to better than 3%,
your optimized stresses can be that far from the design value.
Compare the class file
**
trusswop.a** to trussw.a to
see what you need to do.
For example, the lines:

R,1,(3.0*7.227/16) ! area for elements 1-2
R,2,(3.0*19.86/16) ! area for elements 3-4

change the areas of elements defined after the REAL,1 and REAL,2
commands.
In the formula, 3.0 is the original area, and 7.227 is the original
stress for REAL,1 elements. This line will increase the stress
to 16 ksi. The second line reduces the stress of REAL,1 elements
from 19.86 ksi to 16 ksi.
(Note: Ansys interprets a * as a comment, so be sure to
surround mathematical operations by parentheses.) Then use
REAL,1
E, ... ! elements 1 and 2 in this example
REAL,2
E, ... ! elements 3 and 4 in this example

to change the areas of the appropriate element
When you are done, check that the new stresses match
the design stress.

MENG 421 --
Bridge --
Ansys index --
Class assignments

Last revised: November 19, 2003